Metal vapor laser discharge tubes, although commercially available, have been less than satisfactory when utilized in a machine application. This is paticularly so in the cadmium ion laser that employs helium as an auxiliary gas, the cadmium ion laser emitting light at either 4416 A in the blue region of the spectrum or at 3250 A in the ultraviolet region of the spectrum and capable of producing a continuous wave or pulsed output with modest discharge currents.
Degradation of the radiant output to an unacceptable level is the usual definition of laser tube failure. In turn, the intensity of the output radiation varies with the helium and cadmium pressure and the type and concentration of impurities within the tube. The mechanisms for degradation failure in the helium-cadmium laser tubes are therefore related to depletion of the helium or cadmium supply or accumulation of impurities in the tube.
Depletion of the helium supply is, it is believed, usually a result, inter alia, of permeation through the tube seals and glassware, and trapping by the cadmium condensate. In a typical helium-cadmium laser tube with a consumable cadmium supply, permeation rates may be 0.24 millitorr/hour and helium trapping (or gettering) rates by condensing cadmium may be as high as 38 millitorr/hour.
The source of impurities in laser tubes are both internal and external. Inpurities such as hydrogen will increase the conduction and convection cooling losses from the discharge in the capillary bore and thereby cause alteration of the excited state population distribution in the discharge and the laser output. Water vapor impurities generally dissociate in the tube with the oxygen causing oxidation of the cadmium supply and metal structural components. The residual hydrogen from the dissociation adds to the other sources of hydrogen to cause cooling of the discharge. Internal sources of impurities include absorbed and occluded water vapor and noncondensable gases in the glass walls and structure, evaporated or sputtered electrode or other structural materials, and other internal contaminants resulting from improper cleaning or handling of parts prior to assembly. Suppression or elimination of these impurity problems necessitates that the tube be capable of bake-out at high temperatures (350.degree. to 400.degree. C.) since the contamination problems of most concern are internal to the tube. In those laser tubes using epoxy seals to seal the tube ends, external sources of contamination, generally water vapor and hydrogen may enter the tube thorugh the seals, an epoxy seal being very porous. Many or all of the mechanisms for degradation failure exits in present commercial laser tubes whether in operation or during storage (shelf-life).
The above deficiencies in the prior art metal ion laser discharge tubes have caused a limited acceptance of these tubes in the laser industry. An important use of a helium-cadmium laser, for example, would be in the laser reading/writing systems wherein the blue output of the laser can be effectively utilized as one component of a laser beam which scans (reads) an input document or as a writing beam to write (print) information on a medium sensitive to the blue laser light such as a photoconductor.
Therefore, what is desired in a helium-cadmium laser discharge tube is one which, inter alia, provides an increased lifetime by compensating for helium pressure loss; provides an integral mirror structure to minimize inter-cavity optical surface contaminants; provides a laser mirror seal structure which allows bake-out of the tube at the required high temperature to minimize internal tube contaminants; provides a mirror seal structure which is substantially impermeable to external contaminants; provides diffusion and cataphoretic confinement of the cadmium vapor to protect the laser mirrors; provides a relatively large cadmium reservoir capable of containing sufficient cadmium for operating lifetimes exceeding 10,000 hours; and, allows for control of the cadmium vapor pressure to maintain a constant laser power output independent of ambient environment.
A helium-cadmium laser tube which provides these characteristics is disclosed in copending application Ser. No. 823,553, assigned to the assignee of the present invention. Although the helium-cadmium laser disclosed therein has provided excellent performance, it has been determined that an active control technique for compensating for helium loss would be desirable. Further, certain laser tube design modifications were determined to be desirable in order to increase the efficiency of the laser while reducing its complexity and cost.
It is an object of the present invention to provide an improved metal vapor laser which has increased operating and shelf life times.
It is a further object of the present invention to provide an improved integral mirror helium-cadmium metal vapor laser discharge tube which has substantially increased operating and shelf life times than commercially available laser tubes and which is more efficient and less costly than prior art laser tube designs.
It is still a further object of the present invention to provide an integral mirror helium-cadmium laser tube which includes a cadmium reservoir having a volume sufficient to contain enough cadmium for long operating and shelf life times.
It is an object of the present invention to provide a helium-cadmium laser tube wherein the cadmium vapor pressure is actively controllable such that it is substantially independent of ambient temperatures.
It is still a further object of the present invention to provide a helium-cadmium laser tube wherein the helium vapor pressure is actively controlled by controlling the temperature of a permeable member which is operatively associated with helium gas under high pressure and which is utilized to compensate for helium pressure loss in the tube whereby operating (and shelf) life times are substantially increased over prior art laser tubes.
It is still a further object of the present invention to provide a helium-cadmium laser discharge tube which is economical and has long operating and shelf life times.
It is a further object of the present invention to provide an integral mirror, helium-cadmium laser tube which incorporates both cataphoretic and diffusion confinement sections within the tube for inhibiting cadmium vapor from condensing on the laser mirrors.
It is still a further object of the present invention to provide an integral mirror helium-cadmium laser tube which incorporates a baffle member adjacent one end of the discharge capillary for separating cadmium condensate from the discharge at the cathode electrode for inhibiting trapping, or gettering, of the helium by the cadmium condensate.